This invention relates to a method and apparatus for crimping textile fibers, and particularly to crimping fibrillated polyolefin materials for use as filter materials for tobacco-containing smoking articles.
Texturizing textile fibers, i.e., imparting a crimp into the fibers, using the "stuffer box" principle is well known. Crimping occurs by advancing the fiber at a given rate of speed into an enclosed box whereupon the fiber rapidly decelerates; hence the term stuffing. The stuffer box typically has either a lid that is hinged by one of a variety of techniques, or is fixed with a preset exit orifice, whereby when the box is filled with fiber and a certain predetermined pressure is achieved within the box, fibers will exude out; for example, when the pressure overcomes the forces holding the hinged lid closed, or, if the lid is fixed, when the pressure forces the fiber out the exit orifice.
The effect of the varying pressures inside the box is to impart crimps into the textile fibers. A primary crimp occurs when the individual fibers obtain a wavy shape, for example, during rapid deceleration when the fibers hit the end wall of the stuffer box or the preceding crimped fiber. A secondary crimp occurs when the collapsed individual fibers begin to fold in on themselves inside the stuffer box.
Control of the pressure within the stuffer box is critical because it determines the regularity and nature of the primary crimp, i.e., the crimp frequency and amplitude. Generally, the frequency and amplitude are related so that as the frequency increases, the amplitude decreases. Control of the pressure is generally achieved by careful control of the movement of the hinged lid of the box, or by designing the fixed exit geometry of the box to known preset values such that the pressure drop across the box is known and gives the required fiber crimp characteristics
Although the pressure exerted on the fibers by the hinged lid may be achieved by a variety of techniques, most commercially available systems employ a means of mechanically exerting pressure on the lid to preset values using pneumatic or hydraulic cylinders or actuators, or a known mass (weight) or masses.
Other factors that may affect the nature of the crimp achieved in the textured fiber include the overall geometry and volume of the box and surface frictional characteristics of the internal surfaces of the box, the diameter and surface characteristics of the feed rollers advancing and forcing the fibers into the box and the pressure with which these feed rollers are held together, the temperature of the box, the characteristics of the fibers being crimped and any pretreatment of the fibers. The characteristics of the fibers may vary with, for example, the chemical composition, fiber size and shape, fiber size distribution, number of fibres, and temperature. Pretreatment techniques may include, for example, thermal treatment or adding lubricants, antistatic finishes, oils, moisture, etc.
Conventional stuffer boxes embody a principle relating the width of the stuffer box to the total number of textile fibers or bundles in the tow to achieve desired crimp levels. Generally, for every millimeter of box width the fiber bundle to be crimped should have a size in the range of from about 1000 to about 1800 denier (hereinafter referred to as the "conventional rule"). Accordingly, for a fiber network having a total linear density of 40,000 denier, the stuffer box should have a width in the range of from about 22.2 mm to about 40.0 mm. Conventional equipment not complying with the conventional rule has been found to provide tows having unacceptable crimp characteristics.
It also is known to use fibrillated polyolefin films and form them into filter materials by forming them into a bloomed flocculated mass which is then formed into a filter rod by using conventional filter rod making equipment. These fibrillated polyolefin materials have an interconnected network of fibers and strands, including fibers connected to the network only at one point. These materials have been subjected to the conventional crimping process with some success.
However, one of the problems with the conventional stuffer boxes for crimping fibrillated polyolefin tow is that the crimp amplitude and frequency has not been sufficient to impart adequate filtration capabilities into such materials when formed into filter rods in a conventional manner.
Notwithstanding years of development efforts, there is no commercial use of a filter for smoking articles comprising a fibrillated polyolefin material that provides the advantages and benefits associated with conventional cellulose acetate filter materials used in smoking articles, and particularly, tobacco-containing cigarettes.
Accordingly, there is a continuing need for apparatus and methods for processing fibrillated polyolefin materials to produce filter materials appropriate for use in smoking articles that is more effective, and easier and cheaper to manufacture and form into filters than conventional cellulose acetate materials.